1. Field of the Invention
This invention relates to a process for cooling a hot material to a predetermined temperature as the material is conveyed along a path. In one embodiment, the invention relates to a process for cooling calcined cake.
2. Discussion of the Art
Engineers face a difficult task in designing systems to alter the temperature of materials where the temperature of the material itself cannot be directly measured. Further problems are presented when the amount of material to be cooled cannot be kept constant. Existing systems often result in imprecise control of the material temperature.
Such problems are encountered in, among other places, systems used for cooling calcined coke. Coke is typically formed by the severe heating of a carbonaceous material such as coal or petroleum in an inert atmosphere. After formation, coke which is of sufficient quality can be calcined and further processed to be used in the manufacture of graphite electrodes for the aluminum and steel industries.
Green (uncalcined) petroleum coke is processed by heating it to remove the volatile materials which remain after the coking operation and to change the carbon to hydrogen ratio from about 20:1 to about 1000:1. This calcination typically occurs at temperatures from about 1500.degree. to 3000.degree. F. (815.degree. C. to 1630.degree. C.), preferably from about 2200.degree. to 2600.degree. F. (1200.degree. C. to 1425.degree. C.). Coke exiting the kiln is usually cooled before additional handling or subsequent steps of crushing, mixing with fillers, adding binding agents, and forming.
A conventional means for cooling the coke is a rotary cooler. This is a large cylinder which is slightly inclined from horizontal and slowly rotates such that coke deposited in the upper end will eventually be conveyed by gravity to the opposite end. One or more water quench systems are provided to cool the coke to the desired temperature range and to prevent heat damage to the rotary cooler and related equipment.
Existing process controls have measured the coke temperature at the discharge end of the cooler and have adjusted the rate of a quench spray near the entrance end to keep the discharge coke temperature at the desired value. Changes in the quench spray flowrate can be made to follow significant fluctuations in the coke mass flowrate.
Several problems are found in existing means for cooling coke to a predetermined temperature. One is that the temperature of the coke leaving the kiln and entering the cooler cannot be directly measured because of the physical disturbances during the transfer and the extreme temperatures encountered. Another is that intentional load changes caused by changed output from the kiln require adjustments in the rate of quench water delivered to the cooler. Yet another is that even a constant feed of uncalcined coke to the kiln can produce uneven amounts of coke flow through the kiln and cooler. Still another is that conventional attempts to measure the coke temperature as it exits the cooler and thereby adjust the rate of cooling at the opposite end results in a delayed feedback of the effects of those adjustments, which can produce wide fluctuations in temperature.
If insufficient quench water is used and the coke remains too hot, serious damage to the equipment may result. If too much water is used to cool the coke, it will not meet product specifications. Consequently, it is desirable to conduct the cooling so that coke exiting the cooler is within a specific temperature range.